This invention relates to a magnetic tape recording apparatus such as a VTR in which an FM audio signal and a PCM audio signal are mixed and the resultant audio signal mixture is recorded into a deep portion of a magnetic tape, and then a video signal is recorded over the record of the audio signal mixture.
In some magnetic tape recording apparatuses and VTRs, an FM audio signal and a PCM audio signal are mixed and the resultant audio signal mixture is recorded into a deep portion of a magnetic tape, and then a video signal is recorded over the record of the audio signal mixture.
Japanese published unexamined patent application 1-105301 discloses such a VTR. In the VTR of Japanese application 1-105301, a PCM audio signal undergoes OQDPSK (Off-set Quadrature Differential Phase Shift Keying) modulation before being mixed with an FM audio signal. A mixture of the FM audio signal and the PCM audio signal is superimposed on a high-frequency bias signal, resulting in a final modulated composite audio signal which is recorded into a deep portion of a magnetic tape. Then, a video signal is recorded over the record of the final modulated composite audio signal.
In the VTR of Japanese application 1-105301, during a playback mode of operation of the VTR, cross modulation between the OQDPSK signal and the bias signal causes beats between the two signals which contain a so-called fifth-order distortion having a line spectrum. When the frequency of the cross modulation distortion is near or within the frequency band of the PCM audio signal, the FM audio signal, or a color component of the video signal, the distortion interferes with the latter signal and lowers the quality of the signal at a playback stage.